Bukkit V1.0.0

You can already download the development builds of Craftbukkit from http://ci.bukkit.org - these work with 1.0.0. They are development builds though and are not recommended for use, so use them at your own risk.

I suspect that Legendary Craft are either using one of those, or a version they have made themselves...

There is no recommend build out for Craftbukkkit yet. Only development builds.

They don't have their own version, simply because it can't be done in such a short period (to be FULLY functional, with no errors), because of complicated things
They're just using the bukkit development build!